This proposal is directed towards an understanding of several basic biological processes-energy transduction, binary switching, and organelle assembly-as exemplified in the bacterial flagellum of Salmonella typhimurium. While the proposal is one of basic research, the knowledge gained is used by others who are studying the role of bacterial flagellation and motility in pathogenesis. The first part of the proposal involves an important element of the flagellar motor called the switch. The switch is needed for the flagellum to rotate and for it to control its direction of rotation (counterclockwise vs clockwise) in the behavior called chemotaxis. It is made up from at least three different proteins. Mutations in these proteins, depending on how severe they are, can cause failure of the flagellum to switch, to rotate, or even to assemble. The proposed research will build on existing knowledge of the amino acid changes responsible for these defects, obtained from a large library of spontaneous mutants, and use this knowledge as the basis for exploring in more detail, by systematic deletions and site-directed point mutations, the various aspects of switch function. The second part of the proposal involves the main structural elements of the flagellum, including its helical propeller, and how they are recognized as being distinct from other cellular components in order to be selectively exported. These proteins form a family, whose primary structures are known. They share several features, including periodic repeats of hydrophobic residues and local consensus sequences. These features will be mutated in a selective fashion, and the consequences for export and assembly monitored in an attempt to understand the recognition process.